1. Field of the Invention
The present invention relates generally to electromagnetic energy radiation and reception using a single element antenna, and especially relates to electromagnetic energy radiation and reception with a single element antenna effected using impulse radio energy. Still more particularly the present invention provides a single element antenna suited for broadband energy radiation and reception, and particularly well suited for broadband energy radiation and reception employing impulse radio energy.
2. Related Art
Recent advances in communications technology have enabled an emerging revolutionary ultra wideband technology (UWB) called impulse radio communications systems (hereinafter called impulse radio).
Impulse radio was first fully described in a series of patents, including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. Pat. No. 5,363,108 (issued Nov. 8, 1994) to Larry W. Fullerton. A second generation of impulse radio patents include U.S. Pat. No. 5,677,927 (issued Oct. 14, 1997) to Fullerton et al; and U.S. Pat. No. 5,687,169 (issued Nov. 11, 1997) and U.S. Pat. No. 5,832,035 (issued Nov. 3, 1998) to Fullerton. These patent documents are incorporated herein by reference.
Uses of impulse radio systems are described in U.S. patent application Ser. No. 09/332,502, entitled, xe2x80x9cSystem and Method for Intrusion Detection Using a Time Domain Radar Array, xe2x80x9d and U.S. patent application Ser. No. 09/332,503, entitled, xe2x80x9cWide Area Time Domain Radar Array, xe2x80x9d both filed Jun. 14, 1999, both of which are assigned to the assignee of the present invention, and both of which are incorporated herein by reference.
Basic impulse radio transmitters emit short pulses approaching a Gaussian monocycle with tightly controlled pulse-to-pulse intervals. Impulse radio systems typically use pulse position modulation, which is a form of time modulation where the value of each instantaneous sample of a modulating signal is caused to modulate the position of a pulse in time.
For impulse radio communications, the pulse-to-pulse interval is varied on a pulse-by-pulse basis by two components: an information component and a pseudo-random code component. Unlike direct sequence spread spectrum systems, the pseudo-random code for impulse radio communications is not necessary for energy spreading because the monocycle pulses themselves have an inherently wide bandwidth. Instead, the pseudo-random code of an impulse radio system is used for channelization, energy smoothing in the frequency domain and for interference suppression.
Generally speaking, an impulse radio receiver is a direct conversion receiver with a cross correlator front end. The front end coherently converts an electromagnetic pulse train of monocycle pulses to a baseband signal in a single stage. The data rate of the impulse radio transmission is typically a fraction of the periodic timing signal used as a time base. Because each data bit modulates the time position of many pulses of the periodic timing signal, this yields a modulated, coded timing signal that comprises a train of identically shaped pulses for each single data bit. The impulse radio receiver integrates multiple pulses to recover the transmitted information.
In a multi-user environment, impulse radio depends, in part, on processing gain to achieve rejection of unwanted signals. Because of the extremely high processing gain achievable with impulse radio, much higher dynamic ranges are possible than are commonly achieved with other spread spectrum methods, some of which must use power control in order to have a viable system. Further, if power is kept to a minimum in an impulse radio system, this will allow closer operation in co-site or nearly co-site situations where two impulse radios must operate concurrently, or where an impulse radio and a narrow band radio must operate close by one another and share the same band.
Many applications for impulse radio technology, including communication applications, position determination applications, locating (e.g., radar) applications and other applications require lightweight, compact broadband antennas with broad beam transmit/receive characteristics. As with any antenna, impedance matching to feed elements is necessary for efficient operation. Moreover, in the case of impulse radio technology applications, the antenna must not be subject to ringing in response to application of pulsesxe2x80x94either in a transmit mode or in a receive mode.
Current antenna technology offers several undesirable alternatives to one interested in a small, well-matched, efficient, broad beam ultra wideband (UWB) short pulse antenna: (1) a self-similar antenna (e.g., a spiral antenna) that tends to be large and frequency dispersive; (2) a TEM horn antenna that tends to be bulky and highly directive; or (3) a resistively loaded antenna that will necessarily be lossy and inefficient.
The current art regarding ultra wideband (UWB) antennas teaches using element antennas such as monopoles, dipoles, conical antennas and bow-tie antennas for ultra wideband systems. However, they are generally characterized by low directivity and relatively limited bandwidth unless either end loading or distributed loading techniques are employed, in which case bandwidth is increased at the expense of radiation efficiency.
Conventional antennas are designed to radiate only over the relatively narrow range of frequencies used in conventional narrow band systems. Such narrow band systems may, for example, employ fractional bandwidths no more than about 25%. If an impulse signal, such as a signal of the sort employed for impulse radio purposes, is fed to such a narrow band antenna, the antenna tends to ring. Ringing severely distorts signal pulses and spreads them out in time. Impulse radio signals are preferably modulated by pulse timing, so such distortion of pulses is not desirable.
Broadband antennas are advantageous for many purposes, including their use with impulse radio systems. Conventional design in broadband antennas follows a commonly accepted principle that the impedance and pattern properties of an antenna will be frequency independent if the antenna shape is specified only in terms of angles. That is to say, a self-similar or self-complimentary antenna will be a broadband antenna. This principle explains known broadband antennas like biconical and bow tie antennas, but also applies to other broadband antennas like log periodic, log spiral, and conical spiral antennas.
All such prior art antennas rely on a variation of scale to achieve their broadband performance. A xe2x80x9csmallerxe2x80x9d scale section of the antenna radiates higher frequency components while a xe2x80x9clargerxe2x80x9d scale section of the antenna radiates lower frequency components. Because the radiation centers change location as a function of frequency, these antennas are inherently frequency dispersive; they radiate different frequency components from different parts of the antenna, resulting in a distorted impulse signal.
Throughout this description, it should be kept in mind that discussions relating to transmitting or transmissions apply with equal veracity to reception of electromagnetic energy or signals. In order to avoid prolixity, the present description will focus primarily on transmission characteristics of antennas, with the proviso that it is understood that reception of energy or signals is also inherently described.
A biconical antenna is a classic example of a prior art broadband antenna with an omni-directional pattern. A typical biconical antenna with a 60xc2x0 half angle will have a 100xcexa9 input with a voltage standing wave ratio (VSWR) of  less than 2:1 over a 6:1 bandwidth. A significant drawback with such a biconical antenna is that such an antenna is typically implemented with a diameter equal to the wavelength at the lower frequency limit (xcexL), thus requiring that the antenna be 0.577xcexL in height. Because of similar design limitations, a typical monocone antenna will not provide a good match if it is much less than 0.2xcexL in diameter. In any event, a monocone antenna does not have very stable performance over a broad band. In addition, such large antennas are difficult to fit into a small portable or hand held devices.
TEM horn antennas often suffer from frequency dispersion as well. Furthermore, a horn antenna is inherently a large structure, often several wavelengths in dimension. A horn antenna may be made smaller by dielectric loading, but such loading adds weight which is often undesirable. Further, a horn antenna is a highly directive antenna and cannot provide the less directive coverage required for many portable or mobile applications.
A TEM feed may be combined with a parabolic dish to create a ribbed horn xe2x80x9cimpulse radiating antennaxe2x80x9d (IRA). Such antennas can have bandwidths on the order of a couple of decades, and very high gain, but their large size and high directivity make them inappropriate for portable or mobile use.
Because spherical antennas must be fed by a radial waveguide, they often exhibit poor matching characteristics unless an elaborate and difficult-to-manufacture impedance matching structure is used. A typical impedance matching structure also tends to further impair antenna performance by making the antenna more likely to ring. It is very difficult to construct a feed that maintains a constant matched impedance over a broad bandwidth, something essential to an ultra wideband (UWB) antenna. It is a commonly accepted design criteria in electromagnetic applications, and especially in radio communication applications, that an antenna should match a 50xcexa9 impedance feed providing signals to (or receiving signals from) the antenna. Some video applications require matching a 75xcexa9 impedance feed.
Another prior known antenna structure drives a hemispherical antenna against a ground plane. Attempts by the inventor to employ such an antenna structure for broadband impulse radio resulted in an unacceptably large impedance mismatch.
Circular disc (planar) single element antennas and elliptical disc (planar) single element antennas have been evaluated to determine their respective bandwidths. (Agrawall, Kumar and Ray; xe2x80x9cWide-Band Planar Monopole Antennasxe2x80x9d; IEEE Transactions on Antennas and Propagation, February 1998.) However, no regard was given to the suitability of such antennas for impulse radio applications. No regard was given to dispersion, ringing or phase performance of signals employing such circular disc antennas or elliptical disc antennas for impulse radio communication.
Resistive loading is an alternate technique commonly employed to achieve impedance matching in broadband antennas. Resistive loading succeeds in reducing reflection, but at the cost of throwing away typically around half the power that may be transmitted by an antenna. Such a design trade-off has become accepted in design approaches in prior art antennas. It has been generally believed that resistive loading must be employed for a small broadband antenna in order to achieve good impedance matching. Non-resistively loaded small ultra wideband antennas are known, but they tend to have poor impedance matching and high voltage standing wave ratios (VSWR""s). A lower value for VSWR is a better value; the optimum value of VSWR is 1:1. The prior art teaches that resistive loading must be used in an element antenna in order to achieve wide bandwidth. It is commonly believed that high radiation efficiency and high bandwidth are mutually exclusive.
Other single element planar antennas have been proposed. Henning Harmuth (Henning F. Harmuth, xe2x80x9cNonsinusoidal Waves for Radar and Radio Communication,xe2x80x9d New York: Academic Press, 1981, pp. 108-110.) presented a xe2x80x9clarge current dipolexe2x80x9d (see FIG. 9). The underlying goal of Harmuth""s antenna was to isolate the radiating currents on the loop from currents on the return behind the ground plane.
Variations on Harmuth""s design have been proposed by Pochaninn (Gennadiy P. Pochanin, xe2x80x9cLarge Current Radiator for the Short Electromagnetic Pulses Radiation,xe2x80x9d0 Ultra-Wideband, Short Pulse Electromagnetics Vol. 4, Heyman et. al., ed.; New York: Plenum Publishers, 1999, p. 150.), and Farr (Everett G. Farr, et. al., xe2x80x9cA Two-Channel Balanced-Dipole Antenna (BNA) With Reversible Antenna Pattern Operating at 50 Ohms,xe2x80x9d Sensor and Simulation Notes #441, Air Force Research Laboratory, December 1999. Pochanin suggested employing a ferrite plate as a ground plane and providing a current carrying radiating element with a radius of curvature normal to the ground plane. Farr designed a structure incorporating a radius of curvature normal to the backplane, a broad planar aspect perpendicular with the radius of curvature and a load impedance intermediate the radiating element and the backplane. Both the Pochanin and Farr designs use lossy materialsxe2x80x94Pochanin""s ferrite backplane and Farr""s load impedancexe2x80x94to achieve their design goals. Neither of the Pochanin or Farr antennas uses a simple conducting loop terminated against a ground plane as contemplated by the present invention.
A loop structure has been employed in exciting modes in a waveguide (see, Robert E. Collin, xe2x80x9cField Theory of Guided Waves,xe2x80x9d New York: McGraw-Hill Book Company, 1960, p. 272.) but the structure is limited to a simple wire structure that would not be an effective broadband radiator.
The art, therefore, offers several undesirable alternatives to a small, well-matched, efficient, broad beam ultra wide-band (UWB) short pulse antenna: (1) a self similar antenna tends to be large and frequency dispersive; (2) a TEM horn or Vivaldi slot antenna tends to be bulky and highly directive; (3) a resistively loaded antenna that is lossy and inefficient; or (4) narrow-band devices designed to excite particular waveguide modes
For a small hand held or portable system, it is desirable to have a well matched, efficient, physically small, UWB antenna that radiates non-dispersively and with a broad beam. It is particularly advantageous for an antenna to be easily made in large volumes with reliable repeatable quality. Not only are such antennas unknown to the present art, in fact, the current teaching is that such antennas are not physically realizable.
Larger devices than portable or hand-held products may also enjoy advantages from well matched, efficient and physically small UWB antenna apparatuses. For example, a UWB radar device may be more efficiently, more quickly and more conveniently located for field operations if it involves a compact antenna that exhibits efficient matching and operating characteristics.
An array of single element antennas for use in radar imaging or motion detection should employ single element antennas with patterns that are substantially similar to the desired field of view of the complete radar system. In the particular case of a radar detection device, it is most preferable that single element antennas have a wide field of view in the horizontal direction and a more narrow field of view in the vertical direction. It is also preferable if such radar-employed antenna elements have a horizontal polarity.
There is thus a need for a broadband single element antenna that has ultra wideband response, is compact, is efficiently matched to a feed structure and radiates with a beam that is horizontally polarized and broad in the horizontal direction and more narrow in the vertical direction.
In particular, there is a need for such a broadband single element antenna that operates without ringing in response to application of a pulse signal.
An antenna for transferring electromagnetic energy intermediate a host device and a medium substantially adjacent to the antenna includes (a) a ground element preferably generally coplanar with a ground plane; and (b) a substantially planar transceiver element generally coplanar with a transceiver plane. The transceiver element intersects the ground element at a first end in a joint that has a first terminus and a second terminus. A first edge of the transceiver departs from the first terminus in a first arcuate path in a first direction from the ground plane. A second edge of the transceiver departs from the second terminus in a second arcuate path generally in the first direction. The first edge and the second edge each include at least one arc-set. Each respective arc-set includes a first arc having a first radius describing a respective first edge sector of the first edge and a second arc having a second radius describing a respective second edge sector of the second edge. The first radius and the second radius define a transverse separation between the first edge sector and the second edge sector. The first edge and the second edge terminate in a terminal structure at a second end distal from the first end. The terminal structure is in spaced relation with respect to the ground element to establish a gap intermediate the transceiver and the ground element. The antenna also includes (c) a feed structure. The feed structure conveys the electromagnetic energy intermediate the transceiver and the host device.
Preferably the antenna of the present invention is configured as a layer of copper arranged upon a dielectric substrate to form a generally planar transceiving element that is affixed to a generally planar ground plane. The plane of the transceiver element is preferably substantially perpendicular with the plane of the ground element. The thickness of the dielectric substrate may be advantageously altered to adjust the speed of signal propagation in elements supported by the dielectric material.
An energy guiding means is preferably embodied in a structure that conveys electromagnetic energy. Examples of an energy guiding means include, by way of illustration and not by way of limitation, coaxial cable, stripline, microstrip, twin lead, twisted pair fiber optic cable, wave guide or other transmission line, or a connector or coupler that enables connection to a transmission line.
An energy channeling structure is preferably embodied in a structure that couples electromagnetic energy between an apparatus and an adjacent free space or medium. Examples of a channeling structure include, by way of illustration and not by way of limitation, radiating elements, receiving elements, reflectors, directors and horns.
A transition means is preferably embodied in a structure that receives radio frequency (RF) energy, transmits RF energy or receives and transmits RF energy. The terms xe2x80x9cfeedxe2x80x9d or xe2x80x9cfeed regionxe2x80x9d are sometimes used to refer to a transition means.
A host radio is a RF device that receives RF energy, transmits RF energy or receives and transmits RF energy. An antenna may be integrally included with or within a host radio or that antenna may be situated remotely from the host radio at an arbitrary distance yet coupled with the host radio, such as by using an energy guiding means. The term xe2x80x9chost radioxe2x80x9d does not per se indicate any particular relation between a radio and an associated antenna. In particular, the term xe2x80x9chost radioxe2x80x9d does not preclude an antenna remotely located from a radio or standing alone with respect to a radio.
The term xe2x80x9chost devicexe2x80x9d intentionally indicates an element that may be embodied in other than a radio. Examples of host devices other than radios include, for example, radar devices, location transducer devices, and other devices employing electromagnetic energy transmitted, received or transmitted and received using an antenna.
The present invention is embodied in antennas having a structure characterized by the inventor as xe2x80x9cmonoloopxe2x80x9d antennas. Monoloop antennas are planar single element antennas that are preferably well matched to the standard 50 xcexa9 impedance design parameter employed in communication apparatuses. Monoloop antennas are efficient, physically small and radiate in a broad beam pattern. Such antennas exhibit some spatial dispersion, but they emit a waveform that is relatively short and non-temporally dispersive.
Monoloop antennas generally include a planar radiating loop, a ground plane reflector and a feed structure for providing signals between the antenna and a host device.
A planar radiating loop is preferably a generally planar, approximately semi-circular arc of a suitable conducting material. The plane in which the planar radiating loop is oriented is preferably normal to the plane of the ground plane. The preferred typical shape of the radiating loop is close to circular, but various elliptical, ovoidal, Archimedian and log spiral shapes may also be employed to advantage. It is important to note that the present invention is configured in contrast to teaching of the prior art relating to antenna construction. Rather than being configured to block or minimize reflection from the ground plane, the present invention is oriented to take advantage of the reflections from the ground plane.
The ground plane of the present invention is preferably a suitably conductive sheet that reflects energy from the planar radiating loop. Preferably, the ground plane is a flat conducting plane. A variety of alternate configurations are also useful including a cylindrical reflector, a parabolic reflector, a hyperbolic reflector or a corner reflector.
The feed structure of the present invention includes a transmission line or other energy guiding means, a gap of an appropriate size and a preferably blunt gap intersection or interface intermediate the planar radiating loop and the underlying ground plane. Gap interfaces having smaller diametral dimensions (i.e., less blunt, more pointed configurations) may be employed, but such less blunt gap interface structures present higher input impedance that can be on the order of 100xcexa9-150xcexa9.
It is therefore an object of the present invention to provide an apparatus for transferring electromagnetic energy intermediate a host device and a medium adjacent to the apparatus that is efficient in operation and easy to manufacture in production level quantities.
It is a further object of the present invention to provide an apparatus for transferring electromagnetic energy intermediate a host device and a medium adjacent to the apparatus that is compact and is matched to a feed structure.
It is yet a further object of the present invention to provide an apparatus for transferring electromagnetic energy intermediate a host device and a medium adjacent to the apparatus that radiates a broad beam pattern.
It is a still further object of the present invention to provide an apparatus for transferring electromagnetic energy intermediate a host device and a medium adjacent to the apparatus that operates without ringing in response to application of a pulse signal.